Integrated circuits (“ICs”) are generally manufactured by a process that includes a photolithographic processing step. A photolithographic process step uses photomasks (or reticles) in combination with a light source to optically project a circuit image onto the surface of a silicon wafer or substrate that has a light-sensitive layer, such as photoresist, applied to its surface. A photomask is a transparent ceramic substrate that is coated with a metallic layer forming a pattern for an electronic circuit. During the manufacture of ICs, a pellicle is typically used to seal the photomask from particulate contamination, thereby isolating and protecting the photomask surface from dust or other particles from the focal plane of the photomask pattern.
In order to produce functioning ICs at a high yield rate, the photomask and pellicle need to be free of contamination. Contamination of the photomask can occur both during the manufacture of the photomask itself, and during use of the photomask in the IC manufacturing process, specifically during processing and/or handling of the photomask. One type of contamination is organic/molecular contamination of the photomask surface. Organic/molecular contamination, such as chemical stains or residues, on the surface of the photomask reduces and degrades the transmittance property and/or characteristic of the photomask, ultimately impacting the quality of the semiconductor devices being manufactured.
Another type of contamination that affects the quality of ICs during the photolithography process is particulate contamination. Particulate contamination may include any small particles, such as dust particles, that may be on the photomask or caught between the photomask and the pellicle. Particulate contamination may cause the photolithographic pattern transmitted on the wafer to change, distort, alter, etc. from its intended design, ultimately impacting the quality of the semiconductor device manufactured.
Still another aspect of photomask manufacturing process which is known to affect the quality of the circuit patterns projected during photolithography is the stripping of photoreist from the photomask surface. Similar to the manufacture of the IC devices, during the manufacture of the photomask, photoresist is applied to surface of the photomask and light and/or ultraviolet radiation is applied to the photomask surface in a desired circuit pattern. Once the exposure is completed, the photoresist is removed from the surface of the photomask, thereby revealing the circuit pattern. Proper removal of photoresist is required so as to ensure that the circuit pattern is not changed, distorted, altered, etc. from its intended design.
Because consistent high quality imaging is the goal of every photolithography engineer, substantial efforts go into the proper removal of photoresist during photomask manufacturing and cleaning of the photomask both during photomask manufacturing and as part of their maintenance in IC fabs. Conventional methods employ a high temperature mixture of sulfuric acid and hydrogen peroxide (“SPM”) to strip photoresist, and a high temperature mixture of concentrated ammonium hydroxide/hydrogen peroxide (“APM”) in a second step to further clean the photomask. Typically, the photomask is rinsed with deionized water (“DIW”) after each chemical step and then dried.
As the industry continues to push forward with reduction in IC geometries, the production engineer is required to ensure that lithography maintains performance day after day, lot after lot. Photomasks must consistently print perfect images since this is a major factor affecting device yield. The problem is exasperated by the fact that, photomasks are expensive to purchase and replace. Thus, superior photomask maintenance is one way the lithography engineer can protect high yields and guard the investment that photomasks represent.
Therefore, part of building a robust production system is to implement processes that can clean and return photomasks to production use. Thus, proper cleaning of photomasks is a necessary step that must be incorporated into processing. However, the repetition of conventional cleaning (and stripping) methods have been discovered to deteriorate the life of photomasks.
The goal of the production engineer is more than cleaning photomasks sufficiently to remove contaminants. Extending mask life is also an on-going challenge. Thus, the industry must balance photomask lifetimes against cleaning cycles. Quality cleaning preserves the ability to maintain high device yield while simultaneously extending mask life. Binary and phase shift photomasks, because of their various surface films, require cleaning chemistries strong enough to remove contaminants, yet sensitive enough to avoid damage. Because ICs continue to shrink in size, more stringent cleanliness levels are required. Additionally, new IC manufacturing techniques require advanced photomasks to be designed to allow for small geometries. These advanced masks will see increased cleaning steps and no pellicle protection.
Currently, photoresist cleaning processes are typically done on a single mask spray cleaner tool. Current cleaning techniques include spraying/brushing cleaning, which uses a mixture of SPM followed by ammonia cleans. Problems with the current state of the art processes are that they often suffer from poor chemical mixing or very poor rinsing, resulting in high sulfur content on the surface, which negatively impacts the subsequent mask manufacturing or photolithography processes due to haze. Other problems with state of the art processes are often inconsistent process temperatures due to the way chemicals are heated and introduced to the plate, high rates of chemical consumption, and the fact that brushes tend to induce permanent defects, like scratches, which are tough to remove from the photomask.
Existing methods of stripping photoresist from photomasks during their manufacture use chemistries and recipes similar to those discussed above regarding cleaning in IC fabs. Consequently, current photoresist stripping methods used suffer from many of the same problems and inefficiencies.